假道伐虢
Ask a neighbour for passage to attack a third party — then, positioned inside their territory and with the buffer state gone, turn on the neighbour too. Access granted for one purpose becomes the means of their undoing.
The principle
Two weak states survive by shielding each other; a strong aggressor cannot easily take either while the other stands. The stratagem splits them: persuade one to grant passage — or an alliance, or access — to strike the other, usually with a bribe and the argument that it is not their fight. Once the buffer partner is destroyed and your forces sit inside the accommodating state, the second conquest is trivial. The deep lesson is for the one asked: granting access that removes your own protection is suicide dressed as neutrality. When your neighbour's fall leaves you exposed, their fight is your fight.
Historical origin
Applied today
Beware the partner who wants access to your customers or platform to reach a rival: once inside, with your shared competitor gone, the tooling, data, and relationships you opened up become the wedge they use against you.
When a rival proposes you help them absorb a smaller player 'that isn't your concern,' ask who's next. If that player's fall leaves you exposed, it is precisely your concern — their fight is your fight.
In practice
Invented scenarios showing this stratagem at work — illustrations, not reports of real events.
The integration that learned everything
Access was granted so the partner could sync orders. The scope also revealed pricing patterns, customer concentration and seasonal behaviour across the whole marketplace. Two years later the partner launched a competing service informed by exactly that view. Nothing in the agreement had been breached; the agreement had simply granted passage through the territory.
Use it Scope integrations to the data the function needs — access granted for one purpose teaches everything it touches.
Through the supplier, into the clients
The provider held standing remote-administration access to sixty small businesses that could not afford IT teams of their own. Individually, none of the sixty was worth a targeted campaign. The provider was, and it was reached once. What was borrowed was not data but passage: a route every one of those businesses had agreed to leave open and permanently trusted.
Use it A supplier's access is an inbound route — scope it narrowly, log it separately, and make them evidence their own controls.
The free inspection
The visit was free and unusually thorough. The technician checked the boiler he had come for, then the roof space, the wiring and the drains, and left a nine-item report with three urgencies on it. Two of them were real. Access granted for one small job had quietly surveyed the whole house, and the quotation reflected the tour.
Use it Keep a free visit scoped to the job you booked, and price anything else it uncovers against a second opinion.